Magnetic resonance imaging (MRI) uses the nuclear magnetic resonance (NMR) phenomenon to produce images of patient anatomy. According to MRI techniques, a substance (e.g., human tissue) is subjected to a main polarizing magnetic field, causing the individual magnetic moments of the nuclear spins in the tissue to process about the polarizing field in random order at their characteristic Larmor frequency, in an attempt to align with the field. A net magnetic moment Mz is produced in the direction of the polarizing field, and the randomly-oriented magnetic components in the perpendicular plane (the x-y plane) cancel out one another.
The substance is then subjected to an excitation field (e.g., created by emission of a radiofrequency (RF) pulse) which is in the x-y plane and near the Larmor frequency, causing the net aligned magnetic moment Mz to rotate into the x-y plane so as to produce a net transverse magnetic moment Mt, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The excitation field is terminated and signals are emitted by the excited spins as they return to their pre-excitation field state. The emitted signals are detected, digitized and processed to reconstruct an image using one of many well-known MR reconstruction techniques.
An RF pulse may be emitted as a magnetization preparation step in order to enhance or suppress signals from certain tissue so as to generate desired levels of contrast in the resulting image. For example, an inversion, or saturation, pulse is used in non-contrast-enhanced angiography to suppress venous blood in order to highlight the arterial system.
Systems providing high main magnetic field strengths are increasingly available, with the promise of obtaining images exhibiting ever-higher resolution or Signal-to-Noise ratios. However, emitted RF fields are substantially inhomogeneous at high main magnetic field strengths due to, for example, RF penetration issues and RF coil design issues. The effectiveness of standard RF pulses to selectively enhance or suppress signals is therefore compromised at higher main magnetic field strengths, resulting in degraded image contrast and quality.
In order to address this inhomogeneity, standard RF pulses are conventionally replaced with adiabatic equivalents, such as the hyperbolic secant inversion pulse. Alternative adiabatic equivalents include numerically-optimized adiabatic pulses such as frequency offset corrected inversion (FOCI) pulses, which provide improved slice profiles and tissue suppression at lower RF amplitudes. Unfortunately, the adiabatic performance of an adiabatic pulse is sensitive to changes in its defining pulse parameters such as duration, frequency sweep, slice thickness, etc. Accordingly, an adiabatic pulse which has been numerically-optimized with respect to particular pulse parameters may not function as intended if one or more of those parameters is changed.